The invention relates to a method for determining the amount of charge which can be drawn from a storage battery. The invention also relates to a monitoring device for a storage battery having measurement means for measurement of battery voltages and battery currents, and also having evaluation means.
It may be desirable to estimate the amount of charge which can be drawn from a storage battery during operation.
U.S. Pat. No. 5,761,072 describes a method for determining the capacity of a storage battery for this purpose, in which a filter is used to determine a fast current and a slow current is determined by averaging by means of integration. The values for the fast and slow current are entered in what is referred to as a Peukert relationship in order to determine a capacity for a fast current and for a slow current. These capacities are weighted, and are used to calculate a total capacity.
DE 694 23 918 T2 describes an apparatus for indicating the extent to which a battery is empty, in which measurement values of, for example, the no-load voltage or internal impedance are recorded periodically. These measurement values are filtered via a low-pass filter, and their mean value is determined. If the mean value exceeds a threshold value, an empty warning indication is produced.
DE 691 31 276 T2 discloses an electronic tester for assessing the percentage energy capacity of a battery or of a battery cell. In this method, the dynamic conductance is determined and is compared to a reference conductance, which corresponds to the dynamic conductance of a battery or battery cell with a 100 percent capacity.
These previously known methods and apparatuses may be used to determine the state of charge of a new battery. However, such methods may be desirable for determining the amount of charge which can still be drawn from a used battery, particularly at low current levels.
One difficulty is that the amount of charge which can be drawn from a fully charged storage battery can decrease for various reasons (i.e., it may no longer be possible to draw the same amount of charge from a used storage battery as for a battery in a new state). These reasons may, for example in the case of lead-acid rechargeable batteries, be the loss of active material due to precipitant formation, sulfatation, or the like.
If, by way of example, the state of charge of a storage battery is determined by measuring the no-load voltage, as is possible for example in the case of a lead-acid rechargeable battery, then it is not possible to use this value to make any statement about the amount of charge QR which can still be drawn from the fully charged storage battery when it is no longer new. The reason for this is that, although the state of charge is a measure of the amount of charge which can be drawn from the acid in this case, the amount of charge which can still be drawn from the active material is not correlated with the amount of charge in the acid when new.
If the state of charge (SOC) is defined as the quotient of the difference between the nominal amount of charge and the amount of charge drawn with respect to the nominal amount of charge,   SOC  =                    Nominal  amount  of  charge            -              Amount  of  charge  drawn                    Nominal  amount  of  charge      then the state of charge (SOC) likewise does not provide any information about the amount of charge QR which can be drawn.
Based on this definition, the state of charge provides no information about the actual amount of charge QR which can be drawn from a used storage battery.
U.S. Pat. No. 5,721,688 and U.S. Pat. No. 5,572,136 disclose apparatuses and methods in which a relatively small current which varies with time is applied to a storage battery, and the time-dependent voltage response of the storage battery is observed and evaluated. The conductivity of the storage battery can be determined from the voltage response. However, during operation, it is not always desirable or possible to apply a separate measurement current.
There is thus a need for an improved method for determining the amount of charge QR which can be drawn from a storage battery in the fully charged state. There is also a need for a monitoring device for a storage battery, by means of which it is possible to determine as accurately as possible the amount of charge QR which can be drawn from a used storage battery, using relatively simple means.